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A multi-responsive luminescent sensor based on flexible and ultrastable [email protected] hybrid nanocomposite film
Accepted Manuscript A multi-responsive luminescent sensor based on flexible and ultrastable ZnMOF@SWCNT hybrid nanocomposite film Hong-Yan Lin, Yuan Tian, Shuang Liang, Zi-Wei Cui, Guo-Cheng Liu, Xiu-Li Wang PII: DOI: Reference:
S0277-5387(18)30830-1 https://doi.org/10.1016/j.poly.2018.12.025 POLY 13642
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
9 October 2018 12 December 2018 15 December 2018
Please cite this article as: H-Y. Lin, Y. Tian, S. Liang, Z-W. Cui, G-C. Liu, X-L. Wang, A multi-responsive luminescent sensor based on flexible and ultrastable Zn-MOF@SWCNT hybrid nanocomposite film, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.12.025
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A multi-responsive luminescent sensor based on flexible and ultrastable Zn-MOF@SWCNT hybrid nanocomposite film Hong-Yan Lin,* Yuan Tian,a Shuang Liang,a Zi-Wei Cui,a Guo-Cheng Liua, Xiu-Li Wang* * Department of Chemistry, Bohai University, Jinzhou 121000, P. R. China E-mail: [email protected]; [email protected]
Abstract The application of metal-organic frameworks (MOFs) as a sensing layer has been attracting great interest over the last decade, due to their uniform properties in terms of high porosity and tunability, which provides a large surface area and/or centers for trapping/binding a targeted analyte. Here we report the fabrication of a highly sensitive luminescent sensor that is based on composite thin films of Zn-MOF and single-wall carbon nanotubes (SWCNTs). The approach presented here is facile and paves a promising path towards enhancing the sensitivity of MOF-based sensors. Keywords: Metal-organic framework, Single-wall carbon nanotube, Hybrid nanocomposite, Flexible film, Luminescent sensor
1. Introduction During the last two decades, metal–organic frameworks (MOFs) have been an expanding research area in the field of coordination chemistry not only because of their fascinating architectures and topologies, but also for their potential applications, such as absorption and separation, magnetism, catalysis and sensors.1 Recently, a large number of MOF-based luminescent probes have been synthesized for the detection of divalent transition metal ions.2 As is well known, trivalent metal ions, especially Fe3+, play a vital role in the biological metabolism of cellular systems. Not just for organisms, it is also, more broadly, an environmental contaminant.3 Thus, it has been a hot topic for chemists to detect trivalent metal ions in an accurate way. There are various methods to monitor these metal ions, such as ion mobility spectroscopy
(IMS),
inductively
coupled 1
plasma
(ICP),
X-ray
dispersion,
voltammetry, atomic absorption spectroscopy etc., some of which are relatively limited in their characterization. Current studies indicate that luminescent MOF-based sensors can bring some improvements with their unique properties, such as real-time monitoring, fast response, high sensitivity etc.4 Generally, the networks and properties of MOFs can be influenced by various key factors, such as the metal ions, ligands, molar ratio of solvents, temperature, pH values and so on.5 Among these, ligand selection is highly important for the assembly of MOFs with desired motifs and functions.6 Moreover, it is equally important that the metal ions have a d10 configuration to yield luminescent MOFs. Currently, there are three main reasons for luminescent quenching of MOFs caused by metal ions: (a) interactions between metal ions and organic ligands; (b) collapse of the crystal structure by the metal cations; (c) cation exchange between the central cations of the frameworks and targeted cations. However, many external factors such as temperature, time, and medium could have important effects on the luminescent quenching of MOFs incorporating different metal ions.7 Among them, the medium is heretofore little considered on the metal sensing. Moreover, two or more metal ions combined with specific luminescent materials could and is shown simultaneous quenching in the same system.7d Thus, it is still a great challenge to achieve a detectable signal only for a specific ion, especially for distinguishing variable valency metal ions via luminescent materials. In addition, it is worth noting that most of the developed luminescent materials are powders, and this leads to serious recontamination and recycling problems. From this respect, a freestanding thin film or bulk materials are highly desired, but this usually comes with the problems of a reduction in both detectivity and stability in solutions.8 Therefore, to realize efficient, selective, and reversible luminescent materials using a freestanding bulk material remains a big challenge. Lately, hybrid composites of MOFs have attracted considerable attention for various applications including selective adsorption of metal ions process.9 This field is relatively new and recently several reports on the synthesis and promising applications of the MOF composites are provided. By combining MOFs and 2
appropriate materials, kinetics of synthesis, morphology, physical and chemical properties, stability and potential usage of MOFs can be greatly improved.10 However, the synthesis of MOF-based composites using suitable row materials is crucial. Carbon nanotubes (CNTs) are appropriate materials for synthesis of MOF composites. They suppress MOF aggregation, and control MOF properties including morphology, structure, etc.11 Therefore, MOFs@CNTs composites were hoped to be good luminescent detection materials. In this work, a hybrid nanocomposite based on Zn-MOF and single-walled CNTs (SWCNTs) were synthesized using a simple and facile method. The sufficiency of SWCNT substrate as the main component of the composite to grow nanoscale MOFs was investigated. The properties of synthesized materials were characterized by FT-IR, PXRD and SEM, and were used as a luminescent probe in the detection of Fe3+ ion and pH. Moreover, the film showed tunable shapes and sizes, good flexibility, stiffness, and stability even in solutions (Fig. 1).
Fig. 1 Optical images of Zn-MOF@SWCNT films of different sizes (a), excellent flexibility (b), and different shapes (c). 2. Experimental 2.1. Materials and measurements
3
All reagents were obtained from commercial sources without further purification. The elemental analyses (C, H and N) were determined on a Perkin-Elmer 240C elemental analyzer. The thermal stabilities of the Zn-MOF@SWCNT was analyzed with a thermogravimetric analyzer (NETZSCH STA 449C). FT-IR spectra (KBr pellets) were taken on a Varian FT-IR 640 spectrometer in the range of 500–4000 cm-1. The morphology and structure of the SWCNTs and Zn-MOF@SWCNT hybrid nanocomposites samples were characterized by scanning electron microscopy (SEM, Nova NanoSEM 430). X-ray diffraction (XRD) patterns of powdered samples were recorded at room temperature using a D/Max-2500PC diffractometer with a Cu Kα non-monochromatic radiation source (λ = 1.54056 Å). The specific surface area and pore structure of the samples were investigated with an automatic volumetric sorption analyzer (ASAP 2020 M) using N2 as the adsorbate at −196 oC. Fluorescence spectra were recorded in the solid state at room temperature on a Hitachi F-4500 fluorescence/phosphorescence spectrophotometer. 2.2. Synthesis of materials 2.2.1. Synthesis of [Zn4O(NTB)2]·3DMF·EtOH (Zn-MOF) The Zn-MOF was provided according to the reported procedure with modification.12 The detailed experiment was listed in the supporting information. 2.2.2. Synthesis of single-walled carbon nanotube (SWCNT) The SWCNTs were synthesized according to the literature.13 The detailed experiment was listed in the supporting information. 2.2.3. Synthesis of Zn-MOF@SWCNT hybrid nanocomposite At first, a suspension of SWCNT (10 mg) was prepared in 30 mL N,N-dimethylformamide (DMF)/ethanol (EtOH)/H2O (15/9/6 mL) by sonicating for 30 min. Meanwhile, the 4,4′,4′′-nitrilotrisbenzoic acid (H3NTB) (78 mg) was dissolved in 30 mL DMF/EtOH/H2O (15/9/6 mL). Then, the zinc solution Zn(NO3)2·6H2O (115 mg) was added to the SWCNT suspension and sonicated for 15 min. Afterwards, the H3NTB of DMF/EtOH/H2O mixture solution was slowly added to the solution of zinc solution with SWCNT under constant stirring at room temperature. The final mixture was left, without stirring, which were placed in a 4
Teflon vessel within the autoclave. The mixture was heated at 110 °C for 24 h and then cooled to room temperature. Next, the hybrid nanocomposite was collected by centrifugation, washed several times with fresh EtOH and deionized water and dried at 50 oC in the vacuum oven for 12 h to get the final product. The mass percentage of Zn-MOF in the SWCNTs is characterized by TG analysis (Fig. S1a). TG analysis was performed under an Air atmosphere at a heating rate of 10 °C min−1 in the range of 25 to 1000 °C. The TG curve of Zn-MOF@SWCNT shows two weight loss steps, and the first weight loss of 50.45% in the range of 365–512 °C is consistent with the removal of organic ligand. The second step from 512 to 829 °C can be attributed to the release of the SWCNT. The remaining weight is assigned to ZnO (weight loss of 18.59%). The content of Zn-MOF and SWCNT in the Zn-MOF@SWCNT composite are about 70% and 30%. 2.2.4. Synthesis of Zn-MOF@SWCNT hybrid nanocomposite film Zn-MOF@SWCNT hybrid nanocomposite film was fabricated by a simple filtration method. About 10 mg of Zn-MOF@SWCNT were ultrasonicated in 100 mL of EtOH solution. The suspension was then filtered using a porous cellulose membrane filter with a pore diameter of 0.42 mm. 3. Results and discussion In order to indicate the surface characteristics of hybrid nanocomposites and their precursors, the FT-IR spectra of the prepared materials were studied in the range 4000–400 cm-1. In Fig. 2a, the FT-IR spectrum of SWCNT represented major stretching vibrations at 3413 cm-1 (O–H stretching), 1620 cm-1 (C=C stretching), 1138 cm-1 (C–O stretching) and 1103 cm-1 (C–O–C stretching) as typically reported for SWCNT.14 The adsorption bands at 3067 and 2928 cm-1 are related to the stretching mode of C–H from the aromatic ring and the aromatic ring in H3NTB. The observed peaks in domain of 1545–1399 cm-1 is associated with the vibration of the entire ring and the C–N stretch emerged at 1595 cm-1. The wavenumber region between 1318 and 845 cm-1 shows various bands allocated to the in-plane bending of the ring whereas those observed below 781 cm-1 are attributed to the out-of-plane bending. Also, the absorption band at around 3399 cm-1 ascribed to –OH groups of Zn-MOF. 5
The characteristic peaks of prepared Zn-MOF is assent with the reported data.15 After Zn-MOF was grew on the tube-like structure of SWCNT, the surface chemistry of Zn-MOF@SWCNT was found to include characteristic peaks both from SWCNT and Zn-MOF affirming the successful synthesis of the hybrid nanocomposites. In addition, a precise comparison reveals that several characteristic absorption bands assigned to Zn-MOF, especially for groups located at 1595–845 cm-1, increase with increasing the Zn-MOF content in the nanocomposites structure. The changes in the intensities of the peaks at 1500–1300 cm-1 can be attributed to variation in environment of the H3NTB ligands.14
Fig. 2 (a) FT-IR spectra of Zn-MOF@SWCNT; (b) PXRD patterns of Zn-MOF@SWCNT; SEM images of SWCNT (c) and Zn-MOF@SWCNT (d). The crystalline structures of optimum hybrid nanocomposites and precursors are shown in Fig. 2b. The PXRD pattern of Zn-MOF is identical to the presented patterns confirming the successful preparation of the Zn-MOF nanocrystals. The peak of SWCNT at 2θ = 25.96 corresponds to the (002) reflection of graphite.16 Based on our findings, the highly crystallinity of hybrid nanocomposites indicate that incorporation of SWCNT did not disorganize or destroy the assembly of Zn(II) centers and H3NTB ligands to form Zn-MOF and the MOF crystals grew successfully in the hybrid 6
nanocomposites during in situ synthesis. We also found that the Zn-MOF peaks in the hybrid nanocomposites had lower intensity comparing with that of pure Zn-MOF. This is ascribed to the lower concentration of MOF in the presence of SWCNT substrates.17 As can be seen, the characteristic peaks of SWCNT were not appeared in the pattern of Zn-MOF@SWCNT, which can be attributed to the low content of substrates or the high exfoliation and dispersion of SWCNT in the hybrid nanocomposite.18 The surface morphology and size of parent material and synthesized hybrid nanocomposites was investigated by SEM. The morphology of the SWCNTs and Zn-MOF@SWCNTs were characterized under SEM images is shown in Fig. 2c and 2d. As shown in Fig. 2d, the Zn-MOF nanocomposites with the SWCNTs displayed a thicker diameter than that of the pristine SWCNTs, synthesis of MOF in the presence of SWCNTs resulted in the formation of Zn-MOF ornamented around the SWCNT sidewall. Moreover, the morphology of Zn-MOF@SWCNT nanocomposites tend to be a grape bunch-like. The Brunauer–Emmett–Teller (BET) specific surface area and pore structure of the Zn-MOF@SWCNT film were characterized by nitrogen adsorption–desorption measurements. As shown in Fig. S2a, the isotherm shows adsorption throughout the whole relative pressure range, suggesting a wide pore size range and abundant pore. Analysis using the Barrett–Joyner–Halenda (BJH) equation shows that the mesopore sizes is mainly in the 2–10 nm range (Fig. S2b). The BET surface area and mesopore volume were calculated to be 805 m2 g-1 and 0.15 cm3 g-1, respectively. To evaluate their stability in common organic solvents, Zn-MOF, SWCNT and Zn-MOF@SWCNT were immersed in ethanol for one month at room temperature, respectively, and then the PXRD pattern of the solvent-treated samples was recorded and shown in Fig. S3, which agrees well with the simulated pattern from single crystal analysis, indicating the retention of the framework of Zn-MOF after solvent treatment. Then we collected the PXRD patterns of the samples after treatment in H2O solutions containing 0.01 M of M(NO3)n (M = Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Er3+, Fe3+, Gd3+, K+, Mg2+, Mn2+, Na+, Nd3+, Ni2+, Pb2+, Sr2+, Zn2+ and Zr4+), and H2O 7
solutions with pH values in the range of 1–13 for three days, as shown in Fig. S4. The PXRD profiles are almost unaltered, revealing the retained crystallinity. MOFs including d10 ions have been become potential luminescent materials for wide use in the optical field.19 Therefore, the solid-state luminescence properties of the Zn-MOF and the H3NTB ligand were studied at room temperature (Fig. 3 and S5). The maximum emission peaks were observed at 460 and 457 nm for the Zn-MOF and the H3NTB ligand, respectively. The maximum emission the Zn-MOF is slightly shifted compared to those of the H3NTB ligand, which may arise from ligand-to-metal charge transfer (LMCT).20 In addition, it was reported that photoluminescence can be generated from CNTs.21 There are mainly two proposed mechanisms in understanding this phenomenon. The defect mechanism considers that the trapping of excitation energy at defect sites induced by oxidation leads to emission, while the isolated sp2 carbon cluster mechanism suggests that the isolated sp2 clusters within the sp3 matrix give rise to fluorescence.22 According to the isolated sp2 carbon cluster mechanism, the title CNTs may exhibit similar photoluminescence since the sp2 clusters isolated by sp3 carbon possibly form with the –OH and –COOH onto the surface of CNTs. The solid-state photoluminescence (PL) spectra of the SWCNTs measured under λex = 302 nm exhibited a strong peak at 644 nm (Fig. 3). Moreover, Zn-MOF@SWCNT exhibits two emission bands at 525 and 659 nm with the excitation band at 320 nm, which show about 65 and 15 nm of red-shift comparing with the emission of Zn-MOF and
SWCNT.
The
difference
in
the
luminescence
spectra
between
Zn-MOF@SWCNT and themselves may be attributed to the absence and presence of π–π interactions between the Zn-MOF and SWCNT.
8
Fig. 3 Luminescence spectra of Zn-MOF, SWCNT and Zn-MOF@SWCNT at solid state. The as-synthesized product of Zn-MOF@SWCNT hybrid nanocomposite film was soaked in distilled water with 0.01 M M(NO3)n (M = Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Er3+, Fe3+, Gd3+, K+, Mg2+, Na+, Nd3+, Ni2+, Pb2+, Sm3+, Sr2+, Zn2+ and Zr4+) for 24 h to form the Mn+@Zn-MOF for sensing experiments. The luminescence intensities were increased on the addition of Er3+, Gd3+, Nd3+, Cd2+ and Zn2+ ions in solutions compared to the blank sample (Fig. 4a and S6), while other metal ions showed different quenching effects depending on the nature of the metal ions. Notably for the Fe3+ ion, there is significant quenching behavior in the luminescence of the system. The selective sensing properties were further studied using different mixed ions in an aqueous solution. It was found that the luminescence of Zn-MOF@SWCNT was slightly weakened by mixed ions in the absence of Fe3+ (Fig. 4b). The relationship between the concentration of Fe3+ ions and the luminescence intensities of Zn-MOF@SWCNT was also studied by changing the concentration of Fe3+ ions in aqueous solution in the range of 10-8–10-1 M (Fig. 4c). The titration experiments show that the luminescence of Zn-MOF@SWCNT gradually decreases with increasing concentration of Fe3+. The mechanism of the quenching effect caused by Fe3+ can be explained as an electron transfer from the donor (the organic ligands)
9
to the acceptor (the metal ions). In Zn-MOF@SWCNT, the nitrogen atoms of the H3NTB ligand can act as electrons donors, however, when Fe3+ ions are incorporated with Zn-MOF@SWCNT (Fe3+@Zn-MOF@SWCNT), the lone-pair electrons are given from the nitrogen atoms to the Fe3+ ions to form an electron-deficient region.23 Upon ultraviolet light excitation, the electrons are transferred from the donor to the acceptor, resulting in luminescence quenching. Clearly, the crystalline products of Zn-MOF@SWCNT can be a highly selective sensing probe for Fe3+ based on the above discussions. As illustrated in Fig. S7, the luminescence intensities were all decreased on the addition of metal ions in solutions compared to the blank sample. However, for the Fe3+ ion, there is less significant quenching behavior than the Zn-MOF@SWCNT in the luminescence of the system. It may be attributed to the excellent conductivity of the SWCNT which electrons are fast transferred from the donor to the acceptor, resulting in luminescence quenching.
10
Fig. 4 (a) Luminescence intensity at 525 nm of Zn-MOF@SWCNT in H2O treated with different inorganic anions (10-2 M); (b) The emission spectra of Zn-MOF@SWCNT with different mixed metal ions; (c) Luminescence spectra of Zn-MOF@SWCNT with Fe3+ at different concentrations in a water solution. Insert: Orange balls show luminescence emission at 525 nm at different concentrations, matching the simulated luminescence intensity (blue line). Luminescence pH sensors have certain advantages over electrodes and have widespread applications spanning from environmental analysis and bioanalytical chemistry to medical diagnostics, etc.24 So the pH-dependent luminescence of Zn-MOF@SWCNT has been explored (Fig. 5). The luminescence intensity of 11
Zn-MOF@SWCNT is strongly correlated with the pH value of the dispersed solution in our experimental pH range (1.0–13.0), in which the acidic and basic solution gave weaker luminescence and the weakest intensity was obtained for a pH = 1.0 and 13.0 solution. When the pH value decreases from 7.0 to 1.0, the luminescence intensity of Zn-MOF@SWCNT decreases, the reason for which may derive from the protonation of the nitrogen atoms of the H3NTB ligand. When the pH value increases from 7.0 to 13.0, the luminescence intensity of Zn-MOF@SWCNT decreases, the reason for which may derive from the deprotonation of the –OH groups of the SWCNT. It is noted that the intensity almost decreases linearly along with pH increasing in a pH range (7–1 and 7–13) (Fig. S8) and better than Zn-MOF (Fig. S9). Our preliminary results indicate a pH-dependent luminescence response of Zn-MOF@SWCNT in a wider pH range due to its exceptional chemical stability, which suggests that Zn-MOF@SWCNT could be a promising MOF-based material for pH sensing.
Fig. 5 pH dependent luminescence spectra of Zn-MOF@SWCNT in aqueous solutions with pH ranging from 1.0 to 13.0. Insert: Orange balls show luminescence emission at 525 nm at different pH values, matching the simulated luminescence intensity (blue line). 4. Conclusion Here we report the fabrication of Zn-MOF@SWCNT composite thin films and applied them for luminescent detection. The excellent stability allows the film to be
12
used in an aqueous system, which is highly desirable for practical applications. The results and conclusions of these investigations are summarized as follows: (i) Zn-MOF@SWCNT exhibited a selective fluorescence quenching response to Fe3+ ion with high sensitivity, without the interference of other common metal ions and framework collapse, which can be explained in terms of the competitive absorption of excitation wavelength energy between Fe3+ ion and Zn-MOF@SWCNT; (ii) Zn-MOF@SWCNT shows a relationship between pH value and fluorescence intensity in the pH range from 1 to 13. The results indicate that Zn-MOF@SWCNT have great potential for the development of MOF-based multifunctional luminescent sensing materials. These studies are still underway in our group. Acknowledgements The support of the National Natural Science Foundation of China (No. 21501013) and the Program for Distinguished Professor of Liaoning Province (No. 2015399) is gratefully acknowledged. References [1] (a) H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444; (b) W. H. Zhang, Z. G. Ren and J. P. Lang, Chem. Soc. Rev., 2016, 45, 4995; (c) D. Liu, J. P. Lang and B. F. Abrahams, J. Am. Chem. Soc., 2011, 133, 11042; (d) F. L. Li, Q. Shao, X. Q. Huang and J. P. Lang, Angew. Chem. Int. Ed., 2018, 57, 1888; (e) F. L. Hu, Y. Mi, C. Zhu, B. F. Abrahams, P. Braunstein and J. P. Lang, Angew. Chem. Int. Ed., 2018, 57, 12696; (f) F. Wang, Y. T. Wang, H. Yu, J. X. Chen, B. B. Gao and J. P. Lang, Inorg. Chem., 2016, 55, 9417. [2] (a) H. L. Jiang, Y. Tatsu, Z. H. Lu, and Q. Xu, J. Am. Chem. Soc., 2010, 132, 5586; (b) S. Dang, E. Ma, Z. M. Sun and H. J. Zhang, J. Mater. Chem., 2012, 22, 16920; (c) Y. Li, S. S. Zhang and D. T. Song, Angew. Chem. Int. Ed., 2013, 52, 710; (d) X. H. Zhou, L. Li, H. H. Li, A. Li, T. Yang and W. Huang, Dalton Trans., 2013, 42, 12403; (e) M. J. Dong, M. Zhao, S. Ou, C. Zou and C. D. Wu, Angew. Chem. Int. Ed., 2014, 53, 1575; (f) Y. J. Cui, R. J. Song, J. C. Yu, M. Liu, Z. Q. Wang, C. D. Wu, Y. Yang, Z. Y. Wang, B. L. Chen and G. D. Qian, Adv. Mater., 2015, 27, 1420; (g) T. Y. Gu, M. Dai, D. J. Young, Z. G. Ren and J. P. Lang, Inorg. Chem., 2017, 56, 4668; (h) W. X. 13
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A multi-responsive luminescent sensor based on flexible and ultrastable Zn-MOF@SWCNT hybrid nanocomposite film has been prepared. The approach presented here is facile and paves a promising path towards enhancing the sensitivity of MOF-based sensors.
16
S0277-5387(18)30830-1 https://doi.org/10.1016/j.poly.2018.12.025 POLY 13642
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
9 October 2018 12 December 2018 15 December 2018
Please cite this article as: H-Y. Lin, Y. Tian, S. Liang, Z-W. Cui, G-C. Liu, X-L. Wang, A multi-responsive luminescent sensor based on flexible and ultrastable Zn-MOF@SWCNT hybrid nanocomposite film, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.12.025
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A multi-responsive luminescent sensor based on flexible and ultrastable Zn-MOF@SWCNT hybrid nanocomposite film Hong-Yan Lin,* Yuan Tian,a Shuang Liang,a Zi-Wei Cui,a Guo-Cheng Liua, Xiu-Li Wang* * Department of Chemistry, Bohai University, Jinzhou 121000, P. R. China E-mail: [email protected]; [email protected]
Abstract The application of metal-organic frameworks (MOFs) as a sensing layer has been attracting great interest over the last decade, due to their uniform properties in terms of high porosity and tunability, which provides a large surface area and/or centers for trapping/binding a targeted analyte. Here we report the fabrication of a highly sensitive luminescent sensor that is based on composite thin films of Zn-MOF and single-wall carbon nanotubes (SWCNTs). The approach presented here is facile and paves a promising path towards enhancing the sensitivity of MOF-based sensors. Keywords: Metal-organic framework, Single-wall carbon nanotube, Hybrid nanocomposite, Flexible film, Luminescent sensor
1. Introduction During the last two decades, metal–organic frameworks (MOFs) have been an expanding research area in the field of coordination chemistry not only because of their fascinating architectures and topologies, but also for their potential applications, such as absorption and separation, magnetism, catalysis and sensors.1 Recently, a large number of MOF-based luminescent probes have been synthesized for the detection of divalent transition metal ions.2 As is well known, trivalent metal ions, especially Fe3+, play a vital role in the biological metabolism of cellular systems. Not just for organisms, it is also, more broadly, an environmental contaminant.3 Thus, it has been a hot topic for chemists to detect trivalent metal ions in an accurate way. There are various methods to monitor these metal ions, such as ion mobility spectroscopy
(IMS),
inductively
coupled 1
plasma
(ICP),
X-ray
dispersion,
voltammetry, atomic absorption spectroscopy etc., some of which are relatively limited in their characterization. Current studies indicate that luminescent MOF-based sensors can bring some improvements with their unique properties, such as real-time monitoring, fast response, high sensitivity etc.4 Generally, the networks and properties of MOFs can be influenced by various key factors, such as the metal ions, ligands, molar ratio of solvents, temperature, pH values and so on.5 Among these, ligand selection is highly important for the assembly of MOFs with desired motifs and functions.6 Moreover, it is equally important that the metal ions have a d10 configuration to yield luminescent MOFs. Currently, there are three main reasons for luminescent quenching of MOFs caused by metal ions: (a) interactions between metal ions and organic ligands; (b) collapse of the crystal structure by the metal cations; (c) cation exchange between the central cations of the frameworks and targeted cations. However, many external factors such as temperature, time, and medium could have important effects on the luminescent quenching of MOFs incorporating different metal ions.7 Among them, the medium is heretofore little considered on the metal sensing. Moreover, two or more metal ions combined with specific luminescent materials could and is shown simultaneous quenching in the same system.7d Thus, it is still a great challenge to achieve a detectable signal only for a specific ion, especially for distinguishing variable valency metal ions via luminescent materials. In addition, it is worth noting that most of the developed luminescent materials are powders, and this leads to serious recontamination and recycling problems. From this respect, a freestanding thin film or bulk materials are highly desired, but this usually comes with the problems of a reduction in both detectivity and stability in solutions.8 Therefore, to realize efficient, selective, and reversible luminescent materials using a freestanding bulk material remains a big challenge. Lately, hybrid composites of MOFs have attracted considerable attention for various applications including selective adsorption of metal ions process.9 This field is relatively new and recently several reports on the synthesis and promising applications of the MOF composites are provided. By combining MOFs and 2
appropriate materials, kinetics of synthesis, morphology, physical and chemical properties, stability and potential usage of MOFs can be greatly improved.10 However, the synthesis of MOF-based composites using suitable row materials is crucial. Carbon nanotubes (CNTs) are appropriate materials for synthesis of MOF composites. They suppress MOF aggregation, and control MOF properties including morphology, structure, etc.11 Therefore, MOFs@CNTs composites were hoped to be good luminescent detection materials. In this work, a hybrid nanocomposite based on Zn-MOF and single-walled CNTs (SWCNTs) were synthesized using a simple and facile method. The sufficiency of SWCNT substrate as the main component of the composite to grow nanoscale MOFs was investigated. The properties of synthesized materials were characterized by FT-IR, PXRD and SEM, and were used as a luminescent probe in the detection of Fe3+ ion and pH. Moreover, the film showed tunable shapes and sizes, good flexibility, stiffness, and stability even in solutions (Fig. 1).
Fig. 1 Optical images of Zn-MOF@SWCNT films of different sizes (a), excellent flexibility (b), and different shapes (c). 2. Experimental 2.1. Materials and measurements
3
All reagents were obtained from commercial sources without further purification. The elemental analyses (C, H and N) were determined on a Perkin-Elmer 240C elemental analyzer. The thermal stabilities of the Zn-MOF@SWCNT was analyzed with a thermogravimetric analyzer (NETZSCH STA 449C). FT-IR spectra (KBr pellets) were taken on a Varian FT-IR 640 spectrometer in the range of 500–4000 cm-1. The morphology and structure of the SWCNTs and Zn-MOF@SWCNT hybrid nanocomposites samples were characterized by scanning electron microscopy (SEM, Nova NanoSEM 430). X-ray diffraction (XRD) patterns of powdered samples were recorded at room temperature using a D/Max-2500PC diffractometer with a Cu Kα non-monochromatic radiation source (λ = 1.54056 Å). The specific surface area and pore structure of the samples were investigated with an automatic volumetric sorption analyzer (ASAP 2020 M) using N2 as the adsorbate at −196 oC. Fluorescence spectra were recorded in the solid state at room temperature on a Hitachi F-4500 fluorescence/phosphorescence spectrophotometer. 2.2. Synthesis of materials 2.2.1. Synthesis of [Zn4O(NTB)2]·3DMF·EtOH (Zn-MOF) The Zn-MOF was provided according to the reported procedure with modification.12 The detailed experiment was listed in the supporting information. 2.2.2. Synthesis of single-walled carbon nanotube (SWCNT) The SWCNTs were synthesized according to the literature.13 The detailed experiment was listed in the supporting information. 2.2.3. Synthesis of Zn-MOF@SWCNT hybrid nanocomposite At first, a suspension of SWCNT (10 mg) was prepared in 30 mL N,N-dimethylformamide (DMF)/ethanol (EtOH)/H2O (15/9/6 mL) by sonicating for 30 min. Meanwhile, the 4,4′,4′′-nitrilotrisbenzoic acid (H3NTB) (78 mg) was dissolved in 30 mL DMF/EtOH/H2O (15/9/6 mL). Then, the zinc solution Zn(NO3)2·6H2O (115 mg) was added to the SWCNT suspension and sonicated for 15 min. Afterwards, the H3NTB of DMF/EtOH/H2O mixture solution was slowly added to the solution of zinc solution with SWCNT under constant stirring at room temperature. The final mixture was left, without stirring, which were placed in a 4
Teflon vessel within the autoclave. The mixture was heated at 110 °C for 24 h and then cooled to room temperature. Next, the hybrid nanocomposite was collected by centrifugation, washed several times with fresh EtOH and deionized water and dried at 50 oC in the vacuum oven for 12 h to get the final product. The mass percentage of Zn-MOF in the SWCNTs is characterized by TG analysis (Fig. S1a). TG analysis was performed under an Air atmosphere at a heating rate of 10 °C min−1 in the range of 25 to 1000 °C. The TG curve of Zn-MOF@SWCNT shows two weight loss steps, and the first weight loss of 50.45% in the range of 365–512 °C is consistent with the removal of organic ligand. The second step from 512 to 829 °C can be attributed to the release of the SWCNT. The remaining weight is assigned to ZnO (weight loss of 18.59%). The content of Zn-MOF and SWCNT in the Zn-MOF@SWCNT composite are about 70% and 30%. 2.2.4. Synthesis of Zn-MOF@SWCNT hybrid nanocomposite film Zn-MOF@SWCNT hybrid nanocomposite film was fabricated by a simple filtration method. About 10 mg of Zn-MOF@SWCNT were ultrasonicated in 100 mL of EtOH solution. The suspension was then filtered using a porous cellulose membrane filter with a pore diameter of 0.42 mm. 3. Results and discussion In order to indicate the surface characteristics of hybrid nanocomposites and their precursors, the FT-IR spectra of the prepared materials were studied in the range 4000–400 cm-1. In Fig. 2a, the FT-IR spectrum of SWCNT represented major stretching vibrations at 3413 cm-1 (O–H stretching), 1620 cm-1 (C=C stretching), 1138 cm-1 (C–O stretching) and 1103 cm-1 (C–O–C stretching) as typically reported for SWCNT.14 The adsorption bands at 3067 and 2928 cm-1 are related to the stretching mode of C–H from the aromatic ring and the aromatic ring in H3NTB. The observed peaks in domain of 1545–1399 cm-1 is associated with the vibration of the entire ring and the C–N stretch emerged at 1595 cm-1. The wavenumber region between 1318 and 845 cm-1 shows various bands allocated to the in-plane bending of the ring whereas those observed below 781 cm-1 are attributed to the out-of-plane bending. Also, the absorption band at around 3399 cm-1 ascribed to –OH groups of Zn-MOF. 5
The characteristic peaks of prepared Zn-MOF is assent with the reported data.15 After Zn-MOF was grew on the tube-like structure of SWCNT, the surface chemistry of Zn-MOF@SWCNT was found to include characteristic peaks both from SWCNT and Zn-MOF affirming the successful synthesis of the hybrid nanocomposites. In addition, a precise comparison reveals that several characteristic absorption bands assigned to Zn-MOF, especially for groups located at 1595–845 cm-1, increase with increasing the Zn-MOF content in the nanocomposites structure. The changes in the intensities of the peaks at 1500–1300 cm-1 can be attributed to variation in environment of the H3NTB ligands.14
Fig. 2 (a) FT-IR spectra of Zn-MOF@SWCNT; (b) PXRD patterns of Zn-MOF@SWCNT; SEM images of SWCNT (c) and Zn-MOF@SWCNT (d). The crystalline structures of optimum hybrid nanocomposites and precursors are shown in Fig. 2b. The PXRD pattern of Zn-MOF is identical to the presented patterns confirming the successful preparation of the Zn-MOF nanocrystals. The peak of SWCNT at 2θ = 25.96 corresponds to the (002) reflection of graphite.16 Based on our findings, the highly crystallinity of hybrid nanocomposites indicate that incorporation of SWCNT did not disorganize or destroy the assembly of Zn(II) centers and H3NTB ligands to form Zn-MOF and the MOF crystals grew successfully in the hybrid 6
nanocomposites during in situ synthesis. We also found that the Zn-MOF peaks in the hybrid nanocomposites had lower intensity comparing with that of pure Zn-MOF. This is ascribed to the lower concentration of MOF in the presence of SWCNT substrates.17 As can be seen, the characteristic peaks of SWCNT were not appeared in the pattern of Zn-MOF@SWCNT, which can be attributed to the low content of substrates or the high exfoliation and dispersion of SWCNT in the hybrid nanocomposite.18 The surface morphology and size of parent material and synthesized hybrid nanocomposites was investigated by SEM. The morphology of the SWCNTs and Zn-MOF@SWCNTs were characterized under SEM images is shown in Fig. 2c and 2d. As shown in Fig. 2d, the Zn-MOF nanocomposites with the SWCNTs displayed a thicker diameter than that of the pristine SWCNTs, synthesis of MOF in the presence of SWCNTs resulted in the formation of Zn-MOF ornamented around the SWCNT sidewall. Moreover, the morphology of Zn-MOF@SWCNT nanocomposites tend to be a grape bunch-like. The Brunauer–Emmett–Teller (BET) specific surface area and pore structure of the Zn-MOF@SWCNT film were characterized by nitrogen adsorption–desorption measurements. As shown in Fig. S2a, the isotherm shows adsorption throughout the whole relative pressure range, suggesting a wide pore size range and abundant pore. Analysis using the Barrett–Joyner–Halenda (BJH) equation shows that the mesopore sizes is mainly in the 2–10 nm range (Fig. S2b). The BET surface area and mesopore volume were calculated to be 805 m2 g-1 and 0.15 cm3 g-1, respectively. To evaluate their stability in common organic solvents, Zn-MOF, SWCNT and Zn-MOF@SWCNT were immersed in ethanol for one month at room temperature, respectively, and then the PXRD pattern of the solvent-treated samples was recorded and shown in Fig. S3, which agrees well with the simulated pattern from single crystal analysis, indicating the retention of the framework of Zn-MOF after solvent treatment. Then we collected the PXRD patterns of the samples after treatment in H2O solutions containing 0.01 M of M(NO3)n (M = Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Er3+, Fe3+, Gd3+, K+, Mg2+, Mn2+, Na+, Nd3+, Ni2+, Pb2+, Sr2+, Zn2+ and Zr4+), and H2O 7
solutions with pH values in the range of 1–13 for three days, as shown in Fig. S4. The PXRD profiles are almost unaltered, revealing the retained crystallinity. MOFs including d10 ions have been become potential luminescent materials for wide use in the optical field.19 Therefore, the solid-state luminescence properties of the Zn-MOF and the H3NTB ligand were studied at room temperature (Fig. 3 and S5). The maximum emission peaks were observed at 460 and 457 nm for the Zn-MOF and the H3NTB ligand, respectively. The maximum emission the Zn-MOF is slightly shifted compared to those of the H3NTB ligand, which may arise from ligand-to-metal charge transfer (LMCT).20 In addition, it was reported that photoluminescence can be generated from CNTs.21 There are mainly two proposed mechanisms in understanding this phenomenon. The defect mechanism considers that the trapping of excitation energy at defect sites induced by oxidation leads to emission, while the isolated sp2 carbon cluster mechanism suggests that the isolated sp2 clusters within the sp3 matrix give rise to fluorescence.22 According to the isolated sp2 carbon cluster mechanism, the title CNTs may exhibit similar photoluminescence since the sp2 clusters isolated by sp3 carbon possibly form with the –OH and –COOH onto the surface of CNTs. The solid-state photoluminescence (PL) spectra of the SWCNTs measured under λex = 302 nm exhibited a strong peak at 644 nm (Fig. 3). Moreover, Zn-MOF@SWCNT exhibits two emission bands at 525 and 659 nm with the excitation band at 320 nm, which show about 65 and 15 nm of red-shift comparing with the emission of Zn-MOF and
SWCNT.
The
difference
in
the
luminescence
spectra
between
Zn-MOF@SWCNT and themselves may be attributed to the absence and presence of π–π interactions between the Zn-MOF and SWCNT.
8
Fig. 3 Luminescence spectra of Zn-MOF, SWCNT and Zn-MOF@SWCNT at solid state. The as-synthesized product of Zn-MOF@SWCNT hybrid nanocomposite film was soaked in distilled water with 0.01 M M(NO3)n (M = Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Er3+, Fe3+, Gd3+, K+, Mg2+, Na+, Nd3+, Ni2+, Pb2+, Sm3+, Sr2+, Zn2+ and Zr4+) for 24 h to form the Mn+@Zn-MOF for sensing experiments. The luminescence intensities were increased on the addition of Er3+, Gd3+, Nd3+, Cd2+ and Zn2+ ions in solutions compared to the blank sample (Fig. 4a and S6), while other metal ions showed different quenching effects depending on the nature of the metal ions. Notably for the Fe3+ ion, there is significant quenching behavior in the luminescence of the system. The selective sensing properties were further studied using different mixed ions in an aqueous solution. It was found that the luminescence of Zn-MOF@SWCNT was slightly weakened by mixed ions in the absence of Fe3+ (Fig. 4b). The relationship between the concentration of Fe3+ ions and the luminescence intensities of Zn-MOF@SWCNT was also studied by changing the concentration of Fe3+ ions in aqueous solution in the range of 10-8–10-1 M (Fig. 4c). The titration experiments show that the luminescence of Zn-MOF@SWCNT gradually decreases with increasing concentration of Fe3+. The mechanism of the quenching effect caused by Fe3+ can be explained as an electron transfer from the donor (the organic ligands)
9
to the acceptor (the metal ions). In Zn-MOF@SWCNT, the nitrogen atoms of the H3NTB ligand can act as electrons donors, however, when Fe3+ ions are incorporated with Zn-MOF@SWCNT (Fe3+@Zn-MOF@SWCNT), the lone-pair electrons are given from the nitrogen atoms to the Fe3+ ions to form an electron-deficient region.23 Upon ultraviolet light excitation, the electrons are transferred from the donor to the acceptor, resulting in luminescence quenching. Clearly, the crystalline products of Zn-MOF@SWCNT can be a highly selective sensing probe for Fe3+ based on the above discussions. As illustrated in Fig. S7, the luminescence intensities were all decreased on the addition of metal ions in solutions compared to the blank sample. However, for the Fe3+ ion, there is less significant quenching behavior than the Zn-MOF@SWCNT in the luminescence of the system. It may be attributed to the excellent conductivity of the SWCNT which electrons are fast transferred from the donor to the acceptor, resulting in luminescence quenching.
10
Fig. 4 (a) Luminescence intensity at 525 nm of Zn-MOF@SWCNT in H2O treated with different inorganic anions (10-2 M); (b) The emission spectra of Zn-MOF@SWCNT with different mixed metal ions; (c) Luminescence spectra of Zn-MOF@SWCNT with Fe3+ at different concentrations in a water solution. Insert: Orange balls show luminescence emission at 525 nm at different concentrations, matching the simulated luminescence intensity (blue line). Luminescence pH sensors have certain advantages over electrodes and have widespread applications spanning from environmental analysis and bioanalytical chemistry to medical diagnostics, etc.24 So the pH-dependent luminescence of Zn-MOF@SWCNT has been explored (Fig. 5). The luminescence intensity of 11
Zn-MOF@SWCNT is strongly correlated with the pH value of the dispersed solution in our experimental pH range (1.0–13.0), in which the acidic and basic solution gave weaker luminescence and the weakest intensity was obtained for a pH = 1.0 and 13.0 solution. When the pH value decreases from 7.0 to 1.0, the luminescence intensity of Zn-MOF@SWCNT decreases, the reason for which may derive from the protonation of the nitrogen atoms of the H3NTB ligand. When the pH value increases from 7.0 to 13.0, the luminescence intensity of Zn-MOF@SWCNT decreases, the reason for which may derive from the deprotonation of the –OH groups of the SWCNT. It is noted that the intensity almost decreases linearly along with pH increasing in a pH range (7–1 and 7–13) (Fig. S8) and better than Zn-MOF (Fig. S9). Our preliminary results indicate a pH-dependent luminescence response of Zn-MOF@SWCNT in a wider pH range due to its exceptional chemical stability, which suggests that Zn-MOF@SWCNT could be a promising MOF-based material for pH sensing.
Fig. 5 pH dependent luminescence spectra of Zn-MOF@SWCNT in aqueous solutions with pH ranging from 1.0 to 13.0. Insert: Orange balls show luminescence emission at 525 nm at different pH values, matching the simulated luminescence intensity (blue line). 4. Conclusion Here we report the fabrication of Zn-MOF@SWCNT composite thin films and applied them for luminescent detection. The excellent stability allows the film to be
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used in an aqueous system, which is highly desirable for practical applications. The results and conclusions of these investigations are summarized as follows: (i) Zn-MOF@SWCNT exhibited a selective fluorescence quenching response to Fe3+ ion with high sensitivity, without the interference of other common metal ions and framework collapse, which can be explained in terms of the competitive absorption of excitation wavelength energy between Fe3+ ion and Zn-MOF@SWCNT; (ii) Zn-MOF@SWCNT shows a relationship between pH value and fluorescence intensity in the pH range from 1 to 13. The results indicate that Zn-MOF@SWCNT have great potential for the development of MOF-based multifunctional luminescent sensing materials. These studies are still underway in our group. Acknowledgements The support of the National Natural Science Foundation of China (No. 21501013) and the Program for Distinguished Professor of Liaoning Province (No. 2015399) is gratefully acknowledged. References [1] (a) H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444; (b) W. H. Zhang, Z. G. Ren and J. P. Lang, Chem. Soc. Rev., 2016, 45, 4995; (c) D. Liu, J. P. Lang and B. F. Abrahams, J. Am. Chem. Soc., 2011, 133, 11042; (d) F. L. Li, Q. Shao, X. Q. Huang and J. P. Lang, Angew. Chem. Int. Ed., 2018, 57, 1888; (e) F. L. Hu, Y. Mi, C. Zhu, B. F. Abrahams, P. Braunstein and J. P. Lang, Angew. Chem. Int. Ed., 2018, 57, 12696; (f) F. Wang, Y. T. Wang, H. Yu, J. X. Chen, B. B. Gao and J. P. Lang, Inorg. Chem., 2016, 55, 9417. [2] (a) H. L. Jiang, Y. Tatsu, Z. H. Lu, and Q. Xu, J. Am. Chem. Soc., 2010, 132, 5586; (b) S. Dang, E. Ma, Z. M. Sun and H. J. Zhang, J. Mater. Chem., 2012, 22, 16920; (c) Y. Li, S. S. Zhang and D. T. Song, Angew. Chem. Int. Ed., 2013, 52, 710; (d) X. H. Zhou, L. Li, H. H. Li, A. Li, T. Yang and W. Huang, Dalton Trans., 2013, 42, 12403; (e) M. J. Dong, M. Zhao, S. Ou, C. Zou and C. D. Wu, Angew. Chem. Int. Ed., 2014, 53, 1575; (f) Y. J. Cui, R. J. Song, J. C. Yu, M. Liu, Z. Q. Wang, C. D. Wu, Y. Yang, Z. Y. Wang, B. L. Chen and G. D. Qian, Adv. Mater., 2015, 27, 1420; (g) T. Y. Gu, M. Dai, D. J. Young, Z. G. Ren and J. P. Lang, Inorg. Chem., 2017, 56, 4668; (h) W. X. 13
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A multi-responsive luminescent sensor based on flexible and ultrastable Zn-MOF@SWCNT hybrid nanocomposite film has been prepared. The approach presented here is facile and paves a promising path towards enhancing the sensitivity of MOF-based sensors.
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